A probehead intended to be used for nuclear magnetic resonance measurements in which at least a first and a second distinct kind of nuclear are excited in a homogeneous magnetic field is, for example, disclosed in U.S. Pat. No. 5,229,724 of the same assignee as the present application. This prior art probehead is intended to be used for measurements where the distinct kind of nuclei have highly different resonance frequencies. For example, the first kind of nuclei may be .sup.1 H (protons) having a measuring frequency of 400 MHz when the strength of the homogeneous magnetic field is set to be 9,4 T. The second kind of nuclei, normally referred to as "X" may, for example, be .sup.13 C having a measuring frequency of 100,577 MHz or .sup.15 N having a measuring frequency of 40,531 MHz or .sup.31 P having a measuring frequency of 161,923 MHz.
In order to be able to operate the probehead at these highly different measuring frequencies, distinguishing from each other by at least a factor of two, a radio frequency transmission line configurating an integral component of the probehead, is provided with a switch at half length of the transmission line. The length of the radio frequency transmission line corresponds, for example, to .lambda./2, i.e. half the wavelength of the higher frequency (.sup.1 H).
The so-called X-frequency, i.e. the measuring frequency of the second kind of nuclei may, for certain kinds of nuclei (.sup.15 N) be of about one order of magnitude smaller as compared to the measuring frequency of the first kind of nuclei (.sup.1 H). In other cases (.sup.31 P) it may, however, be of the same order of magnitude as the measuring frequency of the first kind of nuclei. In order to be able to measure the first kind of nuclei as well as the other, the X-kind of nuclei, the switch may be operated accordingly.
It is, further, well-known in the art to perform nuclear magnetic resonance measurements on at least two distinct kinds of nuclei having a measuring frequency that distinguishes only slightly, in particular by only several percents. Examples for such pairs of kinds of nuclei are .sup.1 H having a measuring frequency of 400 MHz and .sup.19 F having a measuring frequency of 376,308 MHz, or, as another example, .sup.31 P having a measuring frequency of 161,923 together with .sup.7 Li having a measuring frequency of 150,454 MHz or, as still another example, .sup.23 Na having a measuring frequency of 105,805 MHz together with .sup.13 C having a measuring frequency of 100,577 MHz, all of the aforementioned frequency values being given relative to a magnetic field strength of 9,4 T.
In an article published in "Journal of Magnetic Resonance", 67, pp. 129-134 (1986), fields of applications are disclosed in which overcritically coupled resonance circuits utilizing discrete components are used. With an overcritical coupling of two resonance circuits, a transmission characteristic is generated, as known per se, having two maxima. In the aforementioned article these maxima are disclosed to be set such that they correspond to the neighbored measuring frequencies of the two kinds of nuclei.
This prior art probehead, however, is suited solely for relatively low frequencies and, due to the discrete components used, cannot be operated within modern spectrometers having a high measuring frequency. Another disadvantage of this prior art probehead resides in the fact that the tuning and the matching of concentrated, discrete components must be effected for both frequencies in the direct vicinity of the measuring coil. This is of particular disadvantage for temperature-controlled measurements because in that case only a very limited space is available.
Further probeheads of the aforementioned kind are disclosed in "Journal of Magnetic Resonance," 72, pp. 168-172 (1987), "Magnetic Resonance in Medicine," 12, pp. 302-309 (1989) and "Journal of Magnetic Resonance", 135, pp. 273-279 (1989).
In a company brochure "BRUKER Instruction Manual for HFX-Unit", Apr. 10.sup.th, 1997, p. 22, still another probehead for nuclear magnetic resonance measurements on two distinct kinds of nuclei having almost the same measuring frequencies, is disclosed. In this prior art probehead, the measuring coil is also connected to a radio frequency transmission line of predetermined length. This system is tuned to the center frequency between the two nuclear magnetic resonance measuring frequencies. At a location somewhat outside half the length of the radio frequency transmission line, a piece of coaxial transmission line of appropriate length is capacitively coupled. By doing so, the center frequency is split. The efficiency of the probehead being tuned to that center frequency, is, however, deteriorated because the energy that is fed to the probehead will be distributed on the two frequencies as well as on a further, still more distant frequency.
Finally, U.S. Pat. No. 5,861,748 discloses still another probehead being described to enable measurements on a plurality of nuclear magnetic resonance measuring frequencies of distinct kinds of nuclei. This prior art probehead consists of a highly branched assembly of coaxial transmission lines of different lengths and different branch points having each distinct matching elements for the various measuring frequencies. This coaxial transmission line assembly of the prior art probehead is disadvantageous because for any change from the one to another kind of nuclei a change to another piece of coaxial transmission line has to be effected for the prevailing lower operational frequency. Moreover, it is highly unlikely that the disclosed coaxial transmission line assembly may be housed in an elongate probehead of small diameter.
It is, therefore, an object underlying the invention to improve a probehead of the kind specified at the outset such that the aforementioned disadvantages are avoided.
In particular, it shall be possible to conduct nuclear magnetic resonance measurements on at least two distinct kinds of nuclei and having highly similar measuring frequencies. In particular, this shall become possible while simultaneously or alternately irradiating and/or observing signals for other kinds of nuclei of a different measuring frequency. The efficiency of the probehead as a whole shall be better than that of prior art assemblies.